USB: Add a sysfs file to show LTM capabilities.

USB 3.0 devices can optionally support Latency Tolerance Messaging
(LTM). Add a new sysfs file in the device directory to show whether a
device is LTM capable. This file will be present for both USB 2.0 and
USB 3.0 devices.

Signed-off-by: Sarah Sharp <[e-mail]>

Documentation/ABI/testing/sysfs-bus-usb

···
 		such as ACPI. This file will read either "removable" or
 		"fixed" if the information is available, and "unknown"
 		otherwise.
+
+What:		/sys/bus/usb/devices/.../ltm_capable
+Date:		July 2012
+Contact:	Sarah Sharp <[e-mail]>
+Description:
+		USB 3.0 devices may optionally support Latency Tolerance
+		Messaging (LTM).  They indicate their support by setting a bit
+		in the bmAttributes field of their SuperSpeed BOS descriptors.
+		If that bit is set for the device, ltm_capable will read "yes".
+		If the device doesn't support LTM, the file will read "no".
+		The file will be present for all speeds of USB devices, and will
+		always read "no" for USB 1.1 and USB 2.0 devices.

drivers/usb/core/hub.c

···
 	return status;
 }
 
-static bool usb_device_supports_ltm(struct usb_device *udev)
-{
-	if (udev->speed != USB_SPEED_SUPER || !udev->bos || !udev->bos->ss_cap)
-		return false;
-	return udev->bos->ss_cap->bmAttributes & USB_LTM_SUPPORT;
-}
-
 int usb_disable_ltm(struct usb_device *udev)
 {
 	struct usb_hcd *hcd = bus_to_hcd(udev->bus);

drivers/usb/core/sysfs.c

···
 }
 static DEVICE_ATTR(removable, S_IRUGO, show_removable, NULL);
 
+static ssize_t
+show_ltm_capable(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	if (usb_device_supports_ltm(to_usb_device(dev)))
+		return sprintf(buf, "%s\n", "yes");
+	return sprintf(buf, "%s\n", "no");
+}
+static DEVICE_ATTR(ltm_capable, S_IRUGO, show_ltm_capable, NULL);
+
 #ifdef	CONFIG_PM
 
 static ssize_t
···
 	&dev_attr_authorized.attr,
 	&dev_attr_remove.attr,
 	&dev_attr_removable.attr,
+	&dev_attr_ltm_capable.attr,
 	NULL,
 };
 static struct attribute_group dev_attr_grp = {

include/linux/usb.h

···
 extern int usb_disable_ltm(struct usb_device *udev);
 extern void usb_enable_ltm(struct usb_device *udev);
 
+static inline bool usb_device_supports_ltm(struct usb_device *udev)
+{
+	if (udev->speed != USB_SPEED_SUPER || !udev->bos || !udev->bos->ss_cap)
+		return false;
+	return udev->bos->ss_cap->bmAttributes & USB_LTM_SUPPORT;
+}
+
+
 /*-------------------------------------------------------------------------*/
 
 /* for drivers using iso endpoints */